1. Field of the invention
The present invention relates to a method for forming a gate of a semiconductor device, and more particularly to a method for forming a gate of a semiconductor device capable of preventing a bridge from being created between gates due to a polymer.
2. Description of the Prior Art
As generally known in the art, the recent tendency towards high integration of semiconductor devices is followed by reduced line width of gates, as well as reduced spacing between gates. As the spacing between gates is reduced, however, a bridge is likely to be created between adjacent gates. This fatally affects the yield rate.
When forming gates, therefore, it is important not only to satisfy the characteristics required by highly integrated devices, but also to prevent a bridge from being created between adjacent gates.
FIGS. 1A and 1B are sectional views showing processes for forming a gate of a semiconductor device according to the prior art.
Referring to FIG. 1A, a gate oxide film 12 is grown on a semiconductor substrate 11 with a thickness of about 54 Å. A poly-Si film 13 and a W film 14 are then deposited on the gate oxide film 12 with a thickness of about 830 Å and 1000 Å, respectively, as gate conductive films.
Subsequently, a SiN film 15 is deposited on the W film 14 with a thickness of about 2100 Å as a hard mask film and a nitride-rich SiON film 16 is deposited on the SiN film with a thickness of about 600 Å as an ARC (anti-reflective coating) film. When the nitride-rich SiON film 16 is deposited, SiH4, N2O, and He are mixed at a flow rate ratio of 70 sccm: 80 sccm: 2200 sccm.
A photosensitive film is then applied on the nitride-rich SiON film 16, which is exposed to light and developed to form a photosensitive film pattern 17 defining a gate formation region.
Referring to FIG. 1B, the photosensitive film pattern is used as an etching mask to etch the nitride-rich SiON film 16 and the SiN film 15 successively. The photosensitive film pattern is then removed in a strip process. Subsequently, the etched SiN film 15 is used as an etching barrier to etch the W film 14, the polySi film 13, and the gate oxide film 12 successively to form a gate 18.
However, the above-mentioned conventional method for forming a gate has a problem as follows:
When the W film and the poly-Si film are etched, in general, a large amount of polymer is created depending on the physical property of the films reacting with the etching gas, i.e., HBr and O2 gases. Particularly, a large amount of nitride component included in the SiON film and the SiN film reacts with the etching gas and creates a large amount of nitride polymer. Such creation of a nitride polymer may be inevitable in a gate etching process requiring high etching selectivity with a gate thin oxide.
As the design rule of devices is reduced to 0.12 μm or less, the spacing between gates decreases and the nitride polymer creates a bridge between adjacent gates. Specifically, the large amount of nitride polymer created by the etching gas and the etching material acts as an etching stop film. As a result, the stable gate etching fails and a bridge is created between adjacent bridges.